The present invention relates to an electrochemical energy accumulator and to a method for switching cells of an electrochemical energy accumulator. The present invention particularly relates to improvements in the actuation of the individual cells for reducing switching losses.
The connection of individual electrochemical storage cells (“cells”) of an electrochemical energy accumulator in series or in parallel in order in each case to achieve desired source characteristics is known from the prior art. A series circuit of electrochemical energy accumulators increases the maximum terminal voltage, whereas a parallel circuit increases the maximum terminal current. In order to satisfy unequal states of charge and states of health of the individual cells of the electrochemical energy accumulator, it is proposed in the prior art that the cells are allowed to participate in the energy output and/or the energy intake of the electrochemical energy accumulator on the basis of probability functions. In this context, the prior art also relates to smart cells which comprise an electrochemical cell, two power semiconductors or respectively power transistors in a half-bridge configuration as well as a cell monitoring circuit having an integrated control unit. FIG. 1 shows a circuit diagram of one such smart cell, also referred to as an intrinsically safe battery cell. A cell monitoring circuit 2 as well as an arrangement of power electronic switches is provided in parallel to cell 3. The latter comprises semiconductor switching diodes 4, 7 connected in series as well as diodes 5,6 which are likewise connected in series and are provided in reverse orientation to the semiconductor switching diodes 4, 7. The semiconductor switching diode 4 and the diode 5 thereby form an upper portion of the half-bridge, the semiconductor switching diode 7 and the diode 6 forming a lower portion of the half-bridge. A connection provided between the semiconductor switching diodes 4, 7 and the diodes 5, 6 forms the output of the smart cell 1 jointly with a connection 9 at the common switching point of the cell 3, the cell monitoring circuit 2 and the lower half-bridge 6, 7. During normal operation, the cell 3 is switched on using the upper portion of the half-bridge 4, 5, whereas the lower portion of the half-bridge 6, 7 is used to switch off the cell 3. As soon as the cell reaches an impermissible state (e.g. maximum minus or minimum voltage or respectively maximum temperature or minimum temperature), whereby the safety of the smart cell 1 is impaired, the upper portion of the half-bridge 4, 5 is switched off, while the lower portion 6, 7 is switched on. An alternative (not depicted) to the smart cell 1 depicted in FIG. 1 uses a full-bridge which makes a reversal of the polarity of the electric voltage possible. In order to achieve a predefined value of a terminal voltage VSet_total, it is proposed according to the prior art that a control unit of the electrochemical accumulator allows one or a plurality of smart cells 1 to be connected in series or in parallel to one another while said smart cells are each switched on with a predefined probability Pon.
FIG. 2 shows a flow diagram to illustrate the generation of the output voltage Vactual_total using a switch-on probability for the smart cell 1. In the flow diagram 10, an actual default value for the voltage Vset_total within a control unit 11 is multiplied by a correction value 12. An additional value is added to the result. The additional value consisting of the difference between the default value Vset_total and an actual terminal voltage value Vactual_total, which is multiplied by a second correction factor 14, is added in an adder 13. A switch-on probability Pon of a plurality 16 of battery cells is added in the result, which leads to the actual voltage value Vactual_total that can be applied to a load 17.
In FIG. 3, a schematic circuit diagram of a series circuit of intrinsically safe battery cells 21, 22, 23, 2n is depicted. A control unit 24 sets defaults for the cells 21, 22, 23, 2n with regard to a switch-on probability Pon and/or a switch-off probability Poff. The control unit 24 receives the terminal voltage Vactual_total dropping across the load 17 as an input variable. It is an aim of the present invention to carry out the operation, in particular the balancing (cell state balancing) of a generic electrochemical energy accumulator faster and more efficiently.